Near-azeotropic ternary compositions constituted by hydrogenated fluorocarbons and hydrocarbons, suitable as refrigerating fluids

ABSTRACT

Near-azeotropic ternary compositions of hydrofluorocarbons with n-butane or isobutane selected from: 
                                   A)   1,1,1,2-tetrafluoroethane (R134a)   65-94%   by weight         pentafluoroethane (R125)   2-25%   by weight         n-butane (R600) and/or isobutane (R-600a)   1-10%   by weight     B)   1,1,1,2-tetrafluoroethane (R134a)   65-94%   by weight         1,1,1,2,3,3,3-heptafluoropropane (R227ea)   5-25%   by weight         n-butane (R600) and/or isobutane (R-600a)   1-10%   by weight                                
Said compositions have zero ODP, very low GWP and good solvent power for mineral lubricants. They are utilizable as drop-in substituents of R-12 in domestic refrigeration.

The present invention relates to near-azeotropic compositions utilizableas refrigerating fluids in circuits working according to the Rankinecycle. More particularly the present invention relates to compositionshaving zero ODP (Ozone depleting Potential) value and low GWP (GlobalWarming Potential) and VOC (Volatile Organic Compound) values,utilizable as low environmental impact substituents ofdichlorodifluoromethane (R-12).

R-12 has been widely used as refrigerating fluid for circuits working atmiddle-high evaporation temperatures, typical of the domesticrefrigeration and of the housing and motorvehicles air-conditioning. Atpresent the manufacturing and marketing of R-12, and generally ofchlorofluorocarbons, are submitted to restrictive rules in all the worldfor applications of this kind because of the alleged destroying power ofthis product on the ozone layer present in the stratosphere.

Therefore, the need of finding other products or compositions capable ofeffectively replacing R-12 without causing environmental damages, isparticularly felt. For this purpose, it was suggested the use ofhydrofluorocarbons (HFC) or hydrochlorofluorocarbons (HCFC), whosedepleting potential with respect to ozone (ODP) is very low or evenzero, as very low is also their contribution to the so called“greenhouse effect”, measured by the GWP.

Among the various substitute products of R-12 up to now proposed, themost known is 1,1,1,2-tetrafluoroethane (R-134a), a product having a lowenvironmental impact (ODP=0); GWP=0.35) and properties similar to thoseof R-12. However R-134a shows as refrigerant a coefficient ofperformance lower than that of R-12 and, on the other side, it is notcompatible with the conventional lubricants of mineral type, thereforeits use as refrigerating fluid requires on the one hand a new planningof the refrigerating circuit to avoid higher energy consumptions and onthe other hand the substitution of the lubricating oil with anothercompatible oil. The oil conventionally used with R-12 is indeed of themineral type while that required by R-134a belongs to the esterifiedpolyols class and the two lubricants are not compatible.

In order to overcome the drawbacks shown by R-134a, alike other singlerefrigerants, the use of mixtures containing HFC has been proposed.However, if mixtures are used, other inconveniences are encountered.First of all, because of the different volatility of the components,fractionation occurs when passing from liquid phase to vapour phase andviceversa, with a remarkable variation in the condensation andevaporation temperatures, so as to impair even considerably theefficiency of the refrigerating circuit. Moreover, the filling up of therefrigerant, necessary in consequence of unavoidable losses from therefrigerating plant, cannot be carried out with the original mixture,but it is necessary to proportion the various components according tothe exact composition of the mixture remained after fractionation, so asto restore the initial optimum composition. Lastly, if the mixturecontains a more volatile, inflammable component, the vapour phaseenriches in such component until the inflammability point is reached,with evident hazard during its use. Similarly, if the inflammablecomponent is less volatile, it concentrates in the liquid phase, givingrise to an inflammable liquid.

In order to avoid such drawbacks, it is therefore convenient to usemixtures having an azeotropic behaviour, i.e. mixtures characterized inthat they behave as pure fluids. However, the obtainment of azeotropicmixtures is an extremely rare event, since it requires a particularcombination of boiling temperatures and deviations from the idealbehaviour of the various components. Therefore, the study ofrefrigerating mixtures has been directed to the obtainment of“near-azeotropic” mixtures. The definition, among those suggested untilnow, which better suits the purposes of the present invention, is thataccording to which a near-azeotropic behaviour occurs if the percentagepressure variation in consequence of a 50% evaporation of the liquid(indicated as Δp/p per 100) at 25° C. is lower than 15% (in this respectsee the article by D. A. Didion and D. B. Bivens in Int. J. Refrig.,vol. 13, p. 163 and following, 1990).

A further characteristic desirable for the substituents ofchlorofluorocarbons (CFC)-based refrigerants, as already mentioned, isthat they shall not virtually require any modifications of elements,materials and, generally, components of the system in which theyoperate: in this case we can speak of “drop-in” substituents. Inparticular, it would be advantageous having a product or a mixturesoluble in the lubricating mineral oils commonly used with conventionalrefrigerants, or soluble in an oil compatible with the oils used atpresent. In such a way, before introducing the new refrigerant, complexoperations of complete discharging, accurate washing and drying of therefrigerating plants would be avoided.

In EP 299614 various near-azeotropic mixtures of halo-carbons areproposed as substituents of R-12 in the refrigerating field. Themixtures of this kind on the one hand still show not zero ODP valuesbecause of the presence of chlorine atoms in one or more components, onthe other hand, according to what ascertained by the Applicant, requirethe use of an alkylbenzenic lubricating oil, with the drawbacksdescribed above.

In EP 565265 are described mixtures containing R-134a, an hydrocarbonselected from propane, propylene or isobutane and optionallyoctafluoropropane (R-218). Although these mixtures are an improvementcompared with R134a used alone, they show the drawback of a notabledeviation from the azeotropic behaviour. See in particular col. 3, lines13-15.

In EP 638623, in the name of the Applicant, mixtures as substituents ofR-12 and R-502 are described, however some of them have not the featureof being “drop-in”, while for others it has been found that they do nothave sufficient chemical stability during the use.

The Applicant has unexpectedly found that HFC-based mixtures containinghydrocarbons as hereinunder defined, have near-azeotropic behaviour, arenon-flammable up to an hydrocarbon content of about 4% by weight or onlyslightly flammable for an hydrocabon content higher than 4% and up to10% by weight and are characterized by vapour pressure curves such as tomake them particularly suitable as substituents for refrigerants R-12,by enjoying moreover of the feature of being “drop-in”. Such mixturesare moreover characterized by a very low or zero environmental impact,expressed in terms of ODP, GWP and VOC.

Therefore, object of the present invention are ternary mixtures,utilizable as refrigerating fluids, essentially of the following types:

A) 1,1,1,2-tetrafluoroethane (R134a) 65-94% by weight pentafluoroethane(R125) 2-25% by weight n-butane (R600) 1-10% by weight B)1,1,1,2-tetrafluoroethane (R134a) 65-94% by weight pentafluoroethane(R125) 2-25% by weight isobutane (R600a) 1-10% by weight C)1,1,1,2-tetrafluoroethane (R134a) 65-94% by weight pentafluoroethane(R125) 2-25% by weight n-butane (R600) and isobutane (R600a) 1-10% byweight D) 1,1,1,2-tetrafluoroethane (R134a) 65-94% by weight1.1,1,2,3,3,3-heptafluoropropane (R227ea) 5-25% by weight n-butane(R600) 1-10% by weight E) 1,1,1,2-tetrafluoroethane (R134a) 65-94% byweight 1,1,1,2,3,3,3-heptafluoropropane (R227ea) 5-25% by weightisobutane (R600a) 1-10% by weight F) 1,1,1,2-tetrafluoroethane (R134a)65-94% by weight 1,1,1,2,3,3,3-heptafluoropropane (R227ea) 5-25% byweight n-butane (R600) and isobutane (R600a) 1-10% by weight,said mixtures having the feature that the percent variation of thevapour pressure after the 50% evaporation of the liquid at thetemperature of 25° C. is comprised between 0.5 and 15% of the vapourpressure before said evaporation and preferably between 0.5 and 7%. Then-butane is usually a commercial product which can contain up to 10% ofisobutane. Similarly, isobutane is usually a commercial product whichcan contain up to 10% of n-butane.

Preferably A, B and C mixtures contain 75-86% of R134a, 4-20% of R-125and 2-4% of hydrocarbon (R-600 and/or R-600a); the D, E and F mixturescontain 75-93% of R-134a, 5-20% of R-227ea and 2-4% of hydrocarbon(R-600 and/or R-600a), since these mixtures result non flammable.Unexpectedly, as already said, the mixtures containing only a littlemore than 4% of hydrocarbon result slightly flammable. Moreover themixtures containing n-butane are preferred to those containingisobutane; this was unexpected as the azeotropic or near-azeotropicbehaviour usually is found more easily when the boiling points of thecomponents are closer.

Generally, the mixtures object of the present invention, beingconstituted by more refrigerants, show the advantage of a greaterflexibility and therefore they meet better than one single component thethermodynamic and thermophysical characteristics required for a certainrefrigerating circuit configuration.

As already said, the above mentioned mixtures are substituents of R-12of drop-in type, as they can be used in the existing equipmentsconcerning refrigeration at middle evaporation temperature, inparticular in the domestic refrigeration, without needing thereplacement of mechanical parts or of conventional mineral lubricatingoils. This feature was unexpected if it is considered that the drop-insubstituents of R-12 previously known contain chlorine. Once dissolvedin the lubricating oil the invention mixtures, unlike those previouslyused, do not cause, also at high temperatures and for long contacttimes, noticeable alterations in the chemical-physical characteristicsof both the oil and the metal surfaces usually present in therefrigerating circuits, revealing therefore a good chemical stability.

Moreover, thanks to the near-azeotropic characteristics, depending onthe content of hydrocarbons, said mixtures show no or at most a slighttendency to fractionate into inflammable liquids or vapours also after asubstantial evaporation of around 50% by weight.

It has also been unexpectedly found that even small percent amounts ofn-butane or isobutane present in these mixtures allow to noticeablyimprove the solubility of conventional mineral lubricants in HFC,notoriously incompatible with these oils. Moreover is has been noticedthat also when the solubility of the oil in the refrigerant is notoptimum, the lubricating oil is unexpectedly capable of coming back fromthe evaporator, which represents the critical zone of the circuit, tothe compressor, thus maintaining lubrication and avoiding wear phenomenaof the compressor mechanical parts and undesired oil accumulations inthe exchangers.

Some working examples of the present invention are hereinunder reported,whose purpose is merely illustrative but not limitative of the scope ofthe invention.

EXAMPLES 1-9 AND 15; COMPARATIVE EXAMPLES 10-14 AND 16-17

Various mixtures according to the present invention were prepared: thecompositions, expressed as % by weight, are reported in Table 1. Eachmixture was characterized according to the following tests:

(a) Near-azeotropic Behaviour

The mixture, of known composition and weight, was introduced into apreviously evacuated small cylinder having an internal volume equal to150 cm³. The filling volume ratio was initially equal to 0.8. Thecylinder was introduced into a thermostatic bath at 25° C. As soon asthe equilibrium was reached, the inner pressure was measured by means ofa pressure transducer. The content of the cylinder was then partlydischarged by means of a suitable valve, until the cylinder weightreached a value corresponding to 50% of the initial charge, by keepingthe temperature at 25° C. The pressure inside the tube was measuredagain at 25° C. The mixture had a near-azeotropic behaviour if thepressure drop, expressed as percentage with respect to the initialpressure (Δp/p·100), was comprised between 0.5 and 15%, preferablybetween 0.5 and 7%. For some mixtures also the value corresponding tothe 90% evaporation of the starting mixture was reported. Although theΔp/p·100 value is not the direct measurement of the azeotropy, it ishowever indicative of an azeotropic behaviour.

In order to more stress the behaviour closer to the azeotropic one ofthe mixtures according to the invention compared with the mixtures ofthe prior art, there was measured the variation of the compositionversus the amount of evaporated liquid at the temperature of 25° C. fora typical mixture according to the invention and for two mixtures ofsimilar composition of the type described in EP 565265. The results arereported in Table 2. The composition of the comparative examples 14 ofTable 1 and 17 of Table 2 practically reproduces the composition ofexample 2 of EP 565265. The improvement represented by the compositionsof the invention results evident.

(b) Boiling Temperature

By using the same cylinder described above, filled with therefrigerating mixture up to a volume ratio of 0.8 and immersed into thethermostatic bath, the boiling temperature is determined by slowlyreducing the temperature of the thermostatic bath until the equilibriumpressure of 1.013 bar is reached: the temperature corresponding to sucha pressure is the boiling temperature of the mixture.

(c) Inflammability

The inflammability of the tested mixtures was determined according to atest which allowed to reveal flame propagation when the test mixture wassupplied onto a burner put at a determined distance.

A burner with oxidizing flame constituted by a Bunsen burner was putnear the zero point of a graduated horizontal rod, so that the thirdupper part of a 5 cm flame was at the same height of the deliveringvalve of a small spray cylinder. The test mixture was introduced in thecylinder and thermostated at the temperature of 20° C. The cylinder wasput at the distance of 15 cm from the burner and the liquid phase of themixture was supplied onto the flame.

The following inflammability evaluation criteria were adopted: themixture was considered non-flammable if no propagation or increase ofthe burner flame was noticed; the mixture was considered slightlyflammable if a slight increase of the flame without propagation wasnoticed; the mixture was considered flammable if propagation of theflame was noticed independently of the flame length. In case of therefrigerating mixtures of the invention the test was carried out on bothliquid phases and on vapor phases at the equilibrium. Before carryingout the test all the mixtures were analyzed by gaschromatographictechnique; the vapour phase of the mixtures was restored as liquid phaseand delivered according to the test modalities.

For comparative purposes the data obtained with the binary mixturesR-134a/R-600 and R-134a/R-600a are reported. The presence of R-125 andR-227ea in the compositions according to the invention allows to improvethe characteristics of non-flammability compared with said binarymixtures.

(e) ODP and GWP

They were calculated on the basis of the known values of the purecomponents constituting the mixture (weighted average), referred toCFCl₃.

EXAMPLE 18 Solubility of Mineral Oil in Refrigerating Mixtures.

A reference mineral oil (SHELL/CLAVUS 32) was introduced in a glass testtube having thick walls resistant to high pressures and closed at oneend by a metal valve. After cooling, the refrigerating mixture to betested was introduced in the test tube previously evacuated and the testtube was immersed in a thermostatic bath. The temperature was firstlyslowly increased from 25° C. up to 60° C. (homogenous solution) and thenreduced until clouding was noticed (cloudy point).

The experimental values obtained with the mixture R-125/R-134a/R-600 inthe 10,90/84, 16/4,94 ratio and for comparative purposes, with R-134aand R-12, are reported in Table 3.

EXAMPLE 19 Wear Test

The test allows to point out possible anomalies in the compressorlubrication by observing the mechanical parts wear. The wear phenomenaare connected to insufficient lubrication caused by poor return of theoil to the compressor or by noticeable decrease of the viscosity of theoil/refrigerant system which is no longer capable of lubricating thecompressor mobile parts.

In a test refrigerant circuit, equipped with an alternative hermeticcompressor for home refrigerators, a liquid mixture consisting ofR-125/R-134a/R-600 in ratio by weight 10:85:5 is introduced. The usedlubricating oil is a mineral oil ISO 32, commonly used with R-12. Thecompressor is kept in continuous working for 1000 hours, by adjustingthe delivery pressure at 20 bar. This period being elapsed thecompressor mechanical parts are submitted to a visual observation, inorder to identify anomalous wear phenomena. The evaluations are carriedout on the basis of a comparative test with the conventional refrigerantR-12. The refrigerant and oil amounts in the two tests are the same. Theresults are reported in Table 4.

EXAMPLES 20 and 21 Tests of Chemical Stability

The mixtures according to the invention were submitted to a chemicalstability test in the presence of metals (copper and steel), accordingto ASHRAE Method 97-1983, with some minor modifications, as reportedhereinafter.

One copper and one steel coupon and about 1 ml of mineral oil such asSHELL CLAVUS 32 were introduced into a glass tube, having a 4.5 mmdiameter and a 250 mm height.

The glass tube was then inserted into a steel cylinder fitted to containexactly the tube, and equipped with a valve. The cylinder was evacuated,cooled and then loaded with 1 ml of refrigerating mixture. The cylinderwas then closed and put in a stove at 175° C. for 14 days.

After such treatment, the refrigerant was analyzed by gaschromatographyto detect the presence of possible by-products deriving from degradationreactions of the refrigerant. The oil was titrated to determine thepossible increase of the acidity (expressed as mg KOH/g oil). The metalcoupons were submitted to visual examination to reveal possible surfacechanges due to corrosion and formation of deposits.

The evaluations are carried out by considering as reference a systemconstituted by R-12/oil/metals. The results obtained with the twomixtures according to the invention are reported in Table 5.

TABLE 1 Near-azeotropic compositions: chemical-physical data VapourNear- Vapour composition in azeotropic tension Density equilibriumbehavior Composition at 25° C. at at 25° C. (ΔP/p 100) B.P.Flammability** Ex. (% by weight) (bar) 25° C. (% by weight) −50% −90% °C. Liq. Vap. ODP*** GWP*** 1 R-125 9.8 7.83 1.14 R-125 17.2 4.72 10.98−30.0 S.F. S.F. 0 0.32 R-134a 85.3 R-134a 76.4 R-600 4.9 R-600 6.4 2R-125 10.4 7.78 1.15 R-125 16.0 5.14 10.92 −29.8 N.F. S.F. 0 0.33 R-134a85.6 R-134a 78.8 R-600 4.0 R-600 5.2 3 R-125 16.6 8.15 1.17 R-125 25.95.77 13.50 −32.0 N.F. N.F. 0 0.36 R-134a 81.0 R-134a 70.8 R-600 2.4R-600 3.3 4 R-125 12.8 7.79 1.17 R-125 20.6 −31.0 N.F. N.F. 0 0.35R-134a 84.6 R-134a 75.8 R-600 2.6 R-600 3.6 5 R-125 6.6 1.17 −29.0 N.F.0 0.31 R-134a 91.0 R-600 2.4 6 R-125 10.6 1.16 R-125 16.4 −33.0 N.F.S.F. 0 0.33 R-134a 86.3 R-134a 78.7 R-600a 3.1 R-600a 4.9 7 R-125 9.17.95 1.14 R-125 143 5.03 12.08 −34.0 S.F. F. 0 0.32 R-134a 85.8 R-134a78.2 R-600a 5.1 R-600a 7.5 8 R-227ea 24.0 6.60 1.18 R-227ea 19.3 2.885.30 −26.0 N.F. S.F. 0 0.40 R-134a 71.0 R-134a 74.0 R-600 5.0 R-600 6.79 R-227ea 9.0 7.35 1.15 R-227ea . . . 4.15 9.98 −32.0 0 0.32 R-134a 86.0R-134a . . . R-600a 5.0 R-600a . . . 10* R-12 100 6.43 1.22 R-12 1000.00 0.00 −29.5 N.F. N.F. 1 2.93 11* R-134a 94.9 1.14 R-134a 93.0 −27.0F. F. 0 0.27 R-600 5.1 R-600 7.0 12* R-134a 95.0 1.14 R-134a 91.5 −32.5F. F. 0 0.27 R-600a 5.0 R-600a 8.5 13* R-218 9.2 8.19 1.15 R-218 17.87.69 15.26 −36.0 0 R-134a 85.8 R-134a 75.7 R-600a 5.0 R-600a 6.5 14*R-218 8.7 9.85 1.10 R-218 17.5 20.81 31.67 −45.0 0 R-134a 85.9 R-134a69.5 R-290 5.4 R-290 13.0 *comparative example **F. = flammable; N.F. =non flammable; S.F. =slightly flammable. ***with respect to CFCl₃

TABLE 2 Variation of composition versus the amount of liquid evaporatedat the temperature of 25° C. Initial composition and after Variation ofthe % Variation of the evaporation of 50 and of 90% liquid compositioncomponents in the by weight (% by weight) (Δ % in peso) liquid ((Δ %/C₁%) C₁ C_((−50%)) C_((−90%)) after 50% after 90% after 50% after 90% Ex.Components liq. vap. liq. vap. liq. vap. evap. evap. evap. evap. 15 R-125 9.8 17.2 5.0 11.8 2.5 2.9 −4.8 −7.3 −49.0 −74.5 R-134a 85.3 76.491.5 82.7 95.4 94.8 +6.2 +10.1 +7.3 +11.8 R-600 4.9 6.4 3.5 5.5 2.1 2.3−1.4 −2.8 −28.6 −57.1 16* R-218 9.2 17.8 3.5 9.6 0.4 1.4 −5.7 −8.8 −62.0−95.6 R-134a 85.8 75.7 92.9 84.5 98.4 96.2 +7.1 +12.6 +8.3 +14.7 R-600a5.0 6.5 3.6 5.9 1.2 2.4 −1.4 −3.8 −28.0 −76.0 17* R-218 8.7 17.5 3.1 9.00.2 0.6 −5.6 −8.5 −64.4 −97.7 R- 134a 85.9 69.5 95.7 86.2 99.7 99.3 +9.8+13.8 +11.4 +16.1 R-290 5.4 13.0 1.2 4.8 0.1 0.1 −4.2 −5.3 −77.7 −98.1*comparative example

TABLE 3 Solubility of mineral oil in refrigerating mixtures Demixingcritical temperature (cloudy point) ° C. Concentration Example 18 of theoil (*) R-125/R-134a/R-600 = % by weight 10,90/84,16/4,94 R-134a R-120.14 +12 +36 <−70 0.20 +45 <−70 0.23 +18 +49 0.30 +24 +58 <−70 0.35 +30<−70 1.94 >+60   2.03 >+60   (*) Olio SHELL/CLAVUS 32

TABLE 4 Wan tests of the compressor (*). Example 19 Mechanical pansR-125/R-134a/R-600 = 10/85/5 R-12 SHAFT: eccentric 1 1 long shank 2 1BODY: cylinder 1 1 hub 1 1 PISTON 1 1 CONNECTING ROD 1 1 VALVE SD SCPLATE VALVE SD SC (*) Analysis codification: 1 Homogeneous slightpolishing without scorings 2 Non homogeneous slight polishing withoutscorings SD Slight deposit SC Slight colouring

TABLE 5 Chemical stability tests (Modified ASHRAE TEST 97-1983 14 daysat 175° C.) (*) Comparison Example 20 Example 21 Refrigerant R-12 100R-125 10.0 R-227ea 24.0 (% by R-134a 85.0 R-134s 71.0 weight) R-600 5.0R-600 5.0 Humidity 13 ppm 15 ppm 15 ppm in the refrigerant Oil mineralmineral mineral Humidity in 35 ppm 35 ppm 35 ppm the oil Metal Cu;AISI-316 Cu; AISI-316 Cu; AISI-316 RESULTS Visual evaluation: Cu B B BAISI-316 U U U oil SC SC SC Acidity (mg KOH/ g oil): before <0.01 <0.01<0.01 after 0.15 0.08 0.09 Gaschroma- 0.18 (**) <0.01 <0.01 tographicanalysis: by-products (%, by weight) (*) Analysis codification: SC =slight colouring U = unaltered B = browning (**) by-product R-22

1. Drop-in substitutes for R 12 consisting of ternary near-azeotropiccompositions: 1,1,1,2-tetrafluoroethane (R 134a) 75-86% by weightpentafluoroethane (R125) 4-20% by weight n-butane (R 600) 2-4% by weightor 1,1,1,2-tetrafluoroethane (R 134a) 75-93% by weight1,1,1,2,3,3,3-heptafluoropropane (R 227ea) 5-20% by weight n-butaneand/or isobutane (R 600 & R 600a) 2-4% by weight

said compositions having the feature that the percent variation of thevapor pressure after 50% percent evaporation of the liquid at atemperature of 25° C. is comprised between 0.5 and 7% of the vaporpressure before said evaporation.
 2. A method of refrigerationcomprising condensing and evaporating the compositions of claim
 1. 3.The drop-in substitutes as claimed in claim 1 wherein the drop-insubstitutes consists of the following ternary near-azeotropiccomposition: 1,1,1,2-tetrafluoroethane (R 134a), pentafluoroethane (R125) and n-butane (R 600).
 4. The drop-in substitutes as claimed inclaim 1 wherein the drop-in substitutes consists of the followingternary near-azeotropic composition: 1,1,1,2-tetrafluoroethane (R 134a),1,1,1,2,3,3,3-heptafluoropropane (R 227ea) and n-butane and/or isobutane(R 600 & R 600a).
 5. A method of refrigeration which comprisesevaporating and condensing a refrigerant mixture which can be obtainedby adding HFC-125 to a mixture of HFC-134a and n-butane, wherein thepresence of HFC-125 causes the flammability characteristics of therefrigerant mixture to be improved as compared to the flammability ofthe mixture of HFC-134a and n-butane.
 6. The method of claim 5, whereinthe n-butane content of the refrigerant mixture is from 2 to 4 wt. %. 7.A method for maintaining lubrication of mechanical parts of a compressorincluded in a refrigeration circuit, which comprises using acompatibilized lubricant-refrigerant mixture formed by combining alubricant with a refrigerant mixture in accordance with claim
 5. 8. Themethod according to claim 7, wherein the lubricant is mineral oil. 9.The method of claim 5, wherein the n-butane content of the refrigerantmixture is from 2 to 4 wt. %.